Drilling a further well or borehole adjacent to already existing wells or boreholes is well-known in the field of oil drilling and production.
In U.S. Pat. No. 3,725,777 a method for determining distance and direction to a cased borehole using magnetic field measurements made from an adjacent borehole is shown. For example in the case of attacking a blowout or drilling multiple wells from a single offshore platform it may be desirable to know the exact location of already existing wells. Such an existing well or borehole is assumed to have a regular periodicity in the casing magnetization. By means of a further assumption of either field data or borehole positions an iterative way of calculation the location of the previously drilled and cased borehole is allowed. However, only final approximate location and field values are obtained.
In EP 247672 a method is disclosed for determining distances between adjacent boreholes. The method of this document is employed in case of a blowout in a previously drilled borehole, a so-called target well, towards which a so-called relief well has to be drilled. In this method accurate magnetic polestrengths values of casing portions have to be known. As a result of complex calculation on Fourier Series of convoluted monopole- and dipole-field functions amplitude/wave number spectra are derived. Such spectra allow determination of the above distances. However, to be capable to apply this method and to obtain such spectra a large number of measurement data is necessary resulting in only mean distances.
Furthermore, from U.S. Pat. No. 4,640,352 it is known to operate a couple of boreholes, the one used as a production well, the other as an injection well. The boreholes were substantially parallel and the problem addressed clearly concerned inducing an economically feasible production of oil from impermeable zones. However, from this document it is not clear how drilling and directing boreholes were carried out.
As to the present invention a quite different problem has to be solved. It will be clear that in the case of a couple of boreholes it may be advantageous to have all the directional data of the production well as soon as the injection well has to be drilled. In such a case it is no longer necessary to previously obtain the data of the first well during drilling the second well.
However, it remains a problem how to control the second borehole drilling operation as accurately as possible and simultaneously to overcome the shortcomings and to avoid the complex operations of the methods as shown above.
Thus, it is an object of the invention to obtain for each depth of a second borehole, when drilled in a predetermined direction adjacent to a first cased borehole having a known position and having casing portions with magnetizations adapted to be measured from the second borehole, a corresponding set of direction drilling data for said second borehole depth.
It is a further object of the invention to prepare directional drilling data immediately employable by the drilling operator for continuing or changing the drilling direction during drilling.
It is yet another object of the invention to check during drilling casing conditions of the first borehole.